Conventionally, in an SOI wafer manufactured by the ion implantation separation method, since an SOI layer is not transferred at its outer circumference, a surface of a base wafer is exposed, thereby forming a terrace. The main reason is that since polishing sag occurs at the outer circumference of the wafer and makes wafer flatness worse, the bonding strength between bonded wafers is low, and the SOI layer is hard to be transferred on the base wafer side.
When the terrace of an SOI wafer is observed with an optical microscope, an SOI island, which is an isolated part of the SOI layer in the shape of an island, is seen at the boundary between the SOI layer and the terrace. It is understood that the SOI island occurs at the transition area between a portion with a degree of flatness that allows the SOI layer to be transferred and a portion with a degree of flatness that does not allow the SOI layer to be transferred. It is feared that the SOI island is detached from the wafer in a device fabrication process, and re-attached to a device fabrication area as a silicon particle, resulting in device faults (See Patent Document 1).
In addition, in ion implantation separation method, since the width of the above terrace (terrace width) is determined depending on flatness of an area in which the terrace will be formed on a wafer to be bonded, it is difficult to control the terrace width after bonding. It is feared that if a laser mark and the like will be made at the terrace of an SOI wafer in a device process, for example, excessively narrow terrace width makes it impossible to make the laser mark.
To improve the SOI island and control the terrace width, there is a method of immersing a wafer after bonding in a solution containing hydrogen fluoride (HF) to etch an insulator film interposed between a bond wafer and a base wafer from the outer circumference (Patent Document 2).